1. Field of the Invention
The present invention relates generally to improvements in downhole drilling motors of the progressive cavity type and, more particularly, pertains to a new improved flexible coupling means between the stator of such a motor and its power output shaft.
2. Description of Related Art
Downhole drilling motors have been used for many years in the drilling of oil and gas wells, for example. In the usual mode of operation, the power output shaft of the motor and the drill bit will rotate with respect to the housing of the motor. The housing, in turn, is connected to a conventional drill string composed of drill collars and sections of drill pipe. This drill string extends to the surface where it is connected to a kelly, mounted to the rotary table of a drilling rig. Drilling fluid is pumped down through the drill string to the bottom of the hole and back up the annulus between the drill string and the wall of the bore hole. The drilling fluid cools the drilling tools and removes the cuttings resulting from the drilling operation. In the instances where the downhole drilling motor is a hydraulic type, such as a progressive cavity type motor, the drilling fluid also supplies the hydraulic power to operate the motor.
Progressive cavity type hydraulic motors are also known as Moineau motors. These hydraulic motors are well known in the art. They have a helical rotor within the cavity of a stator which is connected to the housing of the motor. As the drilling fluid is pumped down through the motor, the fluid rotates the rotor. As the helical rotor rotates, it also gyrates or orbits in the reverse direction relative to its rotation. Some type of universal connection must be used to connect the gyrating rotor to the nongyrating output shaft of the motor. A typical connector utilizes a pair of universal joints which connect a straight rod to the rotor and to the shaft. The universal sections are designed to take only torsional load. A ball and race assembly is used to take thrust load. Rubber boots are clamped over the universal sections to keep drilling fluid out of the ball race assembly. Most assemblies of this type also require oil reservoir systems to lubricate the ball race and universal joints. If the rubber boots loosen or come off, allowing drilling fluid to enter and wear out the ball race assembly, the universal joints are forced to take the torsional and thrust loads, causing premature failure.
Other prior art methods have contemplated the use of long flexible shafts to connect the rotor to the motor output shaft. These shafts flex sufficiently to compensate for the gyration of the rotor. However, in order to provide for sufficient flex, the shafts have to be quite long. As a result, the overall length of the motor becomes excessive. An attempt to overcome this problem is presented in U.S. Pat. No. 4,636,151, issued Jan. 13, 1987 to Jay M. Eppink and assigned to Hughes Tool Company. The invention contemplated in this patent is directed to a connecting rod that has sufficient flexibility without being excessively long.
However, a need still exists to provide for a coupling means which is even shorter than that possible by use of the Eppink invention, and which is even more flexible, so as to permit placement of a bend in the motor housing between its rotor-stator section and the power output shaft section. Such bent housing motors are finding increasing use in steerable directional drilling systems. Such systems are becoming increasingly more important in the field of oil and gas exploration and recovery.